本研究係探討在通電狀態下固態錫鋅合金中金屬原子之電遷移(Electromigration，EM)行為。本研究嘗試以共晶錫鋅銲錫來模擬覆晶接合銲錫接點，於Cu/Sn-9Zn/Cu三明治結構系統中進行通電實驗，藉以瞭解於通電狀態下，電子流對固/固界面反應、富錫相微結構、富鋅相微結構、富鋅相晶格結構(方位)之影響並探討在電遷移作用下富鋅相晶體之成長機制。
界面反應分析結果顯示，電子流造成銅電極之大量消耗，驅使銅原子從陰極端界面往陽極遷移，且在靠近陽極端的基地中形成Cu5Zn8 介金屬化合物(Intermetallic Compound，IMC)，陰極端銅之平均消耗速率較陽極端快。在電遷移之兩極(Polarity)效應作用下，使得-Cu5Zn8、Cu6Sn5化合物在陰、陽兩極端的厚度有所差異。電遷移之兩極效應促進陰極端界面-Cu5Zn8化合物之成核及成長，於室溫、100oC環境下通電(電流密度為1.0 x 103A/cm2)，陰極端Solder/Cu界面-Cu5Zn8化合物的平均成長速率皆較陽極端快，所以在相同的通電時間下，陰極端界面-Cu5Zn8 IMC的厚度皆比陽極端的厚；然而，電遷移之兩極效應卻抑制、減緩了陰極端界面Cu6Sn5化合物之生成，而促進了陽極端界面Cu6Sn5化合物之成核及成長。在電遷移(EM)效應作用下，錫原子被電子驅使往陽極端銅鍍層遷移，沿-Cu5Zn8 IMC晶界擴散進入陽極銅鍍層內部，進而累積在陽極端Cu5Zn8/Cu界面與銅反應形成Cu6Sn5化合物，造成Cu6Sn5化合物往陽極端銅鍍層內部成長。
由於界面Cu5Zn8 IMC之快速成長以及電遷移效應作用所誘發之壓縮應力，會導致在銲錫基地的中間區域形成錫晶粒之突起。於通電狀態下，電子流會誘發界面-Cu5Zn8 IMC的大量生成，以及陰、陽兩極端IMC/Solder界面大量孔洞之形成，在陰極端所形成的孔洞具相當大的尺寸且呈棒狀(Rod-like)；在陽極端界面所形成的為較小尺寸的針狀孔洞。然而，於時效熱處理狀態下，在IMC/Solder界面則發現非常少的孔洞。
於通電前，散佈於銲錫合金基材中之第二相(Second Phase)-富鋅相(Zn-rich Phase)無規則地(Randomly)分佈於銲錫基地中，經100oC熱處理24小時或經130oC時效熱處理230小時後，富鋅相晶粒皆等軸(Equiaxial)成長；而經長時間通電處理後，富鋅相晶粒之組織(Texture)方位會趨向與電子流方向一致，且富鋅相晶粒之形態(Morphology)為長方形薄片(Rectangular Sheet)或菱形薄片(Prism Sheet)。
X-ray繞射分析結果顯示，於通電狀態下，電子流會誘發富鋅相晶體內原子之重新排列(Reorientation)，隨著通電時間的增加，富鋅相晶粒(0002)面方位的訊號峰逐漸增強，富鋅相晶體之(0002)峯(沿c軸方向)的相對強度遠大於(10 0)峯(垂直c軸方向)的相對強度(通電50、100、230小時後，比值( )分別為3.1、5.2、18.9)，此乃表示電遷移效應導致富鋅相晶體由原本等軸晶轉變成具非常強的(0002)之優選方位(Preferred Orientation)甚至於單晶(Single Crystal)結構；然而，富鋅相晶體經時效熱處理230小時，並無特殊之優選方位。
穿透式電子顯微鏡(TEM)分析結果顯示，經長時間通電處理後，等軸富鋅相晶體發生再結晶(Recrystallization)並趨向形成單晶結構，以最密堆積基面(Basal Plane，(0001)或(0002)平面)排列，且其最密堆積平面與電子流方向平行。在電遷移效應作用下，時效處理後之富鋅相多晶晶體(Poly-crystal)再結晶形成具(0001)優選方位之菱形薄片富鋅相晶體，而此晶體乃由六方(Hexagonal)鋅晶體單位晶胞(Unit Cell)之基面所組成。通電狀態下，非契合界面(Incoherent Interface)的成長速率大於半契合界面(Semicoherent Interface)，導致生成菱形薄片結構富鋅相晶體。在電遷移效應與非契合界面成長效應之聯合作用下，造成富鋅相晶粒(單晶)沿二維方向成長。

The purpose of this research is to investigate the electromigration (EM) behavior of metallic atoms in Sn-9Zn alloy. It also investigated the effect of electromigration on the interfacial reaction, the microstructural evolution of Sn-rich phase and Zn-rich phase, and the lattice orientation of the Zn-rich crystals in a Cu/Sn-9Zn/Cu sandwich structure. The reorientation and recrystallization behaviors of dispersed Zn second phases in Sn-9Zn alloy upon electromigration was also proposed in this study.
Electromigration resulted in the consumption of Cu from the cathode which migrated to the anode and formed Cu5Zn8 near the anode in bulk solder. The average consumption rate of Cu on the cathode side was much faster than that of Cu on the anode side. The occurrences upon electromigration (EM) shows significant polarity effect on the nucleation and growth rate of the IMCs (-Cu5Zn8, Cu6Sn5). Upon current stressing, the growth rate of the Cu-Zn intermetallic compound (-Cu5Zn8) at the cathode interface was much faster than that at the anode. However, the nucleation and growth of the Cu6Sn5 IMC at the anode interface were enhanced, though retarded at the cathode, under the influence of electric current. The growth of the Cu6Sn5 phase, which nucleated at the anodic Cu5Zn8/Cu interface into the anodic Cu layer, was ascribed to the grain-boundary migration of Sn induced by electromigration. Consequently, the anodic Cu6Sn5/Cu interface was induced to move toward the anodic Cu due to electromigration.
The formation of hillocks in the middle of bulk solder was ascribed to the compressive stress driven by electromigration and IMC formation. The electromigration results in void formation at the IMC/solder interface regardless of the electron flow direction. There is difference in the morphology and dimension of voids at the two ends of the solder (cathode and anode). Relatively large voids (rod shape) were formed at the cathode, yet small needle-like voids were formed at the anode. Nevertheless, fewer void was seen at the solder/IMC interface after thermal aging.
Before current stressing, the fine dispersed Zn-rich phases randomly distribute in the matrix. The needle Zn-rich phase of the Sn-9Zn eutectic structure converts to equi-axial crystals after aging at 100oC for 24 hours (As prepared specimen) or at 130oC for 230 hours. Notably, the random texture of the Zn-rich phase crystals became laterally aligned after current stressing for 230 hours. The electrical current stressing converts the equi-axial Zn grains to longitudinal thin sheet or thin prism sheet crystals.
XRD analysis showed that the reorientation of the second phase Zn-rich crystals was induced by electric current. The Zn-rich phase grew with a preferred orientation of (0002) upon current stressing. The remarkable difference in peak intensity between (0002) and (10 0) after 230 hours of current stressing showed that the thin sheet prism crystals of the Zn phase exhibit preferred orientation or even grow to single crystal. However, the Zn phase exhibits both (10 0) and (0002) lattice planes with close intensity upon aging at 130oC for up to 230 hours.
TEM analysis showed that electromigration resulted in the recrystallization of the equiaxial Zn-rich phase crystal into a single crystal composed of the close-packed basal plane of the hexagonal Zn phase. The Zn poly-crystal of aged Sn-9Zn alloy recrystallized under electromigration to form a thin prism sheet crystal of basal plane unit with (0001) preferred orientation. The mobility of the semicoherent interface is much smaller than that of the incoherent interface. Consequently, the electromigration of Zn within the Zn phase gave rise to the formation of the thin prism sheet crystals. The electromigration behavior induced the continuing movement of the Zn atoms towards the incoherent interface which synergizes the two dimensional growing behavior of the grains and thus facilitated the formation of prism sheet crystals with the preferred [0001] orientation.

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